Roses seem to be high-maintenance plants, subject to both disease and insect attack. When aphids or other insects are spotted, it's tempting to immediately reach for the sprayer. However, a light infestation can often be dislodged by a strong spray of water. If you do need an insecticide, a systemic that moves through the plant's tissues from the roots to the leaves can be a wise choice.
Systemic insecticides are designed to kill insects that suck or chew on plant tissue. In essence, they turn the rose or other shrub into a poisonous substance. Systemic insecticides are applied to the soil, absorbed by the roots and transferred to the leaves and other soft tissue by the movement of fluid within the plant. Each proprietary, brand-name insecticide has its own formula containing one of the many systemic insecticides as an active ingredient. Some of those available include acephate, imidacloprid, dinotefuran and disulfoton.
Ease of application is a primary advantage of systemic insecticides. No sprayer or mixing is needed. Another benefit is the lack of drift onto other areas of the garden, especially during windy days. Being contained within the plant, these will not kill beneficial insects unless they also consume some of the leaves or shoots.
New insecticides are constantly being introduced and old ones phased out. Products containing dimethoate, for instance, are now banned. Read the label of any insecticide and know the active ingredients. Different chemicals may vary in effectiveness against aphids, thrips, Japanese beetles and other insects.
Some formulas contains both fertilizer and systemic insecticide but, since the need for each may be different at different times of the year, it is best to use two separate products rather than a combination.
Systemic insecticides are best applied in spring so the developing leaves and shoots can absorb the pesticide. However, unless you have reasonable cause to suspect an infestation, it's best to wait. A few aphids or thrips aren't going to damage your roses significantly, and preventative applications of insecticide can just be a waste of money. Always carefully follow the directions on the container for safe and effective applications.
Like all pesticides, systemic insecticides are toxic substances and need to be treated carefully. Always protect your skin from contact, wash your hands after use and keep the product away from pets and children. Whenever possible, use products that deteriorate quickly to avoid groundwater contamination and harm to wildlife.